While many have speculated that a missile may have struck a Russian commercial airliner that went down over Egypt's Sinai peninsula, U.S. officials are now saying satellite imagery doesn't back up that theory.

A senior defense official told NBC News late Monday that an American infrared satellite detected a heat flash at the same time and in the same vicinity over the Sinai where the Russian passenger plane crashed.

According to the official, U.S. intelligence analysts believe it could have been some kind of explosion on the aircraft itself, either a fuel tank or a bomb, but that there’s no indication that a surface-to-air missile brought the plane down.

The Metrojet-operated Airbus A321 crashed Saturday in Egypt's Sinai peninsula, killing all 224 on board. The local ISIS affiliate claimed responsibility for shooting it down — but officials dismissed that as impossible by saying the militants lacked the weapons to do so.

And U.S. Director of National Intelligence James Clapper told a "Defense One" conference Monday that there was not yet any "direct evidence of terrorist involvement" in the crash.